Indirect light-emitting type device which displays images by controlling the transmittance of an external light source, the backlight unit, which is the light source and an important component which determines the characteristics of LCD.
Particularly, as LCD panel manufacturing technology has developed, the demand for LCD displays having high brightness has increased, and thus there have been various attempts to increase the brightness of the backlight unit. Thus, in the case of liquid crystal displays which are used as monitors, personal digital assistants (PDAs), notebook computers and the like, the display of bright light from a low-energy source is used to measure their superiority. Accordingly, in the case of an LCD, the front brightness is very important.
In an LCD, light passed through the light diffusion layer is diffused in all directions because of the structure of the LCD, and thus light which is displayed on the front side is very insufficient. For this reason, there has been a continued effort to display high brightness with low power consumption. In addition, efforts are being made to increase the viewing angle of LCD displays by increasing the area of the displays so as to enable more users to view the displays.
When the power of the backlight unit is increased for this purpose, the power consumption becomes larger and the power loss by heat also becomes larger. For this reason, in the case of portable displays, the battery capacity becomes larger and the battery life-cycle becomes shorter.
Accordingly, methods of imparting directionality to light in order to increase brightness have been suggested, and for this purpose, various lens sheets have been developed. A typical prism sheet has a prism array structure in which a number of peaks and valleys are linearly arranged side by side.
The prism structure has a triangular array structure having an inclined plane of about 45 in order to improve the brightness in the front direction. Thus, there are problems in that, because the upper portion of the prism structure is in the shape of a peak, the upper part of the peak is readily broken or worn away by a small external scratch, thus causing damage to the prism structure. Because the angle coming from the prism structures of the same type is the same between the arrays, a small collapse occurs in the triangular corner portion, or a fine scratch occurs in the inclined plane. Thus, due to the difference in the path for emitting light between the damaged area and the normal area, brightness is reduced and failure occurs. Accordingly, in some cases, an entire side of produced prism sheets cannot be used due to fine defects occurring during the production of prism sheets. This leads to a decrease in productivity and an increase in production cost. Actually, in companies which manufacture backlight unit assemblies, a failure caused by damage to the prism structure by a scratch during handling of the prism sheets is a significant problem.
Also, an operation for stacking several sheets and films is carried out during the manufacture of a backlight unit. In this operation, a plurality of prism films can be stacked in order to increase brightness. In this case, the upper portion of the lower prism film comes in contact with the lower portion of the upper prism film, thus causing a problem in that the prism structure is readily damaged.
In addition, it is a general tendency to apply structured optical sheets in view of an increase in brightness, hiding or viewing angle. When these optical sheets are mounted on the backlight unit, an operation of stacking them with other sheets or films is carried out. In this case, the upper portion of the lower prism film comes in contact with the lower portion of the upper prism film, and thus the prism structure can be readily damaged. Also, care is required in conveying such sheets or handling them in processes.
Accordingly, in order to prevent damage to such prism structures, a protective film was stacked in some cases. However, because LCD panels gradually become thinner, it is a general tendency to omit the protective film or use a sheet having combined functions. In addition, adding the process of stacking the protective film leads to an increase in production cost and a decrease in time efficiency and physical efficiency.
In addition to damaging the prism structure by handling of the prism sheet during the manufacture of backlight units, while the use of portable displays such as notebook computers and PDAs is increasing, the case in which the displays are placed in bags during movement is rapidly increasing. In this case, when impact is applied to the displays by running or a sudden stop during movement, the prism structures placed in the displays are damaged even when there is a protective film on the prism structures, thus affecting the displays.
For this reason, there is an urgent need for an optical sheet which can flexibly cope with external impact.